NBSIR-74-364

DETECTION OF HUMAN INTRUDERS BY LOW FREQUENCY

SONIC INTERFEROMETRIC TECHNIQUES

Robert E. Stoltenberg

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

May 1974

Final Report

Prepared for

Sandia Laboratories

U.S. Atomic Energy Commission

Order No. AL-73-356

Albuquerque, New Mexico

NBSIR-74-364

DETECTION OF HUMAN INTRUDERS BY LOW FREQUENCY

SONIC INTERFEROMETRIC TECHNIQUES

Robert E. Stoltenberg

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

May 1974

Final Report

Prepared for

Sandia Laboratories

U.S. Atomic Energy Commission

Order No. AL-73-356

Albuquerque, New Mexico

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS. Richard W Roberts Director

CONTENTS

Page

1.0 INTRODUCTION 1

1.1 Background 1

1.2. Concept 2

1.2.1 Ambient Acoustical Noise 5 1.2.2 Noise Caused by Enviromental Changes 5

1.2.3 Equipment Noise and Sensitivity-- 6

2.0 OBJECTIVE 7

3.0 INSTRUMENTATION REQUIREMENTS 8

3.0.1 Ambient Noise Characteristics 8

3.0.2 Target Scattering (reflection coefficient) 8

3.0.3 Sound Pressure Level Field Patterns 8

3.1 Ambient Sound Pressure Level Measurements 9

3.2 Target Scattering Calculations 9

3.3 Sound Pressure Level Field Pattern Measurements 12

3.4 Conclusions 15

4.0 INSTRUMENTATION 17

4.1 Sound Source 17

4.1.1 Transducer 17

4.1.2 Amplifier and Oscillator 18

4.2 Microphones and Preamplifiers -- 18

4.3 Acoustical Hardware Mounting 18

4.4 Receivers 18

4.4.1 Functional Analysis 19

4.5 Recording Capabilities and Signature Analysis- 23

5.0 TEST LOCATION DESCRIPTIONS 24

5.1 Room 3082 , Radio Building 24

5.2 NBS Auditorium 24

5.3 NBS Instrument Shop 27

5.4 Cameo Building #5 27

iii
CONTENTS (Continued)

Page

6.0 PRELIMINARY EVALUATION, ROOM 30 82

SPECTRAL DISTRIBUTION OF INTRUDER SIGNATURE AND

NOISE 2 7

6.1 Test Description 30

6.2 Test Results and Analysis 30

6.3 Conclusions 36

7.0 INTRUDER TESTS, NBS AUDITORIUM 38

7.1 Test Description 38

7.2 Test Results and Analysis 39

7.2.1 Intruder Signature Analysis 39

7.2.2 Sonification Level vs. Noise and Intruder Signature Magnitudes 41

7.2.3 Intruder Signature as a Function of

Source and Receiver Characteristics 44

7.2.4 Detection as a Function of Geometry 48

7.2.5 Spectral Distribution vs. Intruder

Velocity 48

7.2.6 Intruder Size Comparison 48

7.2.7 Additional Exploratory Tests 52

7.3 Conclusions 55

8.0 INTRUDER TESTS, NBS INSTRUMENT SHOP 56

8.1 Test Descriptions 56

8.2 Test Results and Analysis 57

8.2.1 Intruder Signature Analysis 57

8.2.2 Sonification Level vs. Noise and Intruder Signature Magnitude 57

8.2.3 Intruder Signature as a Function of

Source Characteristics 60

8.2.4 Detection as a Function of Geometry 62

8.2.5 Intruder Signature as a Function of

Intruder Velocity and Size 62

8.3 Conclusions 66

iv
CONTENTS (Continued)

Page

9.0 WAREHOUSE FEASIBILITY TESTS 66

9.1 Test Descriptions 66

9.2 Test Results and Analysis 67

9.3 Conclusions 70

10.0 LONG TERM FALSE ALARMS 70

10.1 Test Descriptions 72

10.2 Test Results and Analysis 72

10.3 Conclusions 73

11.0 CONCLUSIONS 73

11.1 Conclusion by Parameter 73

11.1.1 Intruder Signature 74

11.1.2 Background Noise 74

11.1.3 Geometry of the Source and Receiver 75

11.1.4 Area of Coverage 75

11.1.5 The Effects of Source and Microphone

Characteristics 75

11.1.6 General Comments 76

11.2 Recommendations 76

11.2.1 Minimum Velocity Effects 76

11.2.2 Additional Evaluation of Sound

Pressure Fields-- 76

11.2.3 Design and Construction of Prototype

Engineering Model Detectors 76

11.2.4 Detection Capability and False Alarm

Tests 77

11.2.5 Engineering Model Detector and Test 77

ACKNOWLEDGMENTS 7 7 REFERENCE 7 7 APPENDIX 78

v
LIST OF ILLUSTRATIONS

Page

Figure 1.1 Vectorial signal addition 3

Figure 3.1 Ambient sound pressure level 10

Figure 3.2 Normalized scatter from a rigid sphere 11

Figure 3.3 Typical sound pressure level spatial

distribution 13

Figure 3.4 Circular sound pressure patterns 14

Figure 4.1 Receiver block diagram 20

Figure 5.1 Room 3082 plan view 25

Figure 5.2 Auditorium plan view 26

Figure 5.3 Instrument shop plan view 28

Figure 5.4 Cameo Building plan view 29

Figure 6.1 Spectral display carrier frequency dependence 31

Figure 6.2 Corrected spectral display 33

Figure 6.3 Receiver output vs. correlation 34

Figure 6.4 Target return vs. carrier frequency 37

Figure 7.1 Intruder signature, spectral dependence upon

carrier frequency 40

Figure 7.2 Sonification vs. noise level and intruder

signature 42

Figure 7.3 Sonification vs. noise and intruder signature 45

Figure 7.4 Intruder signature microphone/preamp vs.

sound level meter 46

Figure 7.5 Intruder signature speaker vs. horn 47

Figure 7.6 Detection vs. geometry 49

vi
LIST OF ILLUSTRATIONS (Continued)

Page

Figure 7.7 Intruder signature vs. intruder velocity 50

Figure 7.8 Intruder size comparison 51

Figure 7.9 Radio effects upon detection 53

Figure 7.10 Random intruder actions 54

Figure 8.1 Intruder signature spectral dependence vs.

carrier frequencies 58

Figure 8.2 Target strength as a function of carrier

frequency 59

Figure 8.3 Intruder signature as a function of source 61

Figure 8.4 Detection vs. microphone position 63

Figure 8.5 Single speaker-single microphone detection 64

Figure 8.6 Intruder signature as a function of size and

velocity 65

Figure 9.1 Intruder signature vs. geometry 68

Figure 9.2 Target strength vs. carrier frequency 69

Figure 9.3 Sonification vs. noise and intruder signature 71

Figure A.l Microphone amplifier, speaker amplifier, and

DC-DC converter schematics 79

Figure A. 2 Receiver wiring diagram 80

Figure A. 3 Amplitude control board 3.4, 5.0 kHz 81

Figure A. 4 Amplitude control board 1.0, 1.7, 2.2 kHz 82

Figure A. 5 Voltage variable phase shifter 83

Figure A. 6 Quadrature hybrid 84

Figure A. 7 Amplitude detector 85

Figure A. 8 Phase detector 86

Figure A. 9 Receiver response 87

vii

DETECTION OF HUMAN INTRUDERS

BY

LOW FREQUENCY SONIC INTERFEROMETRI C TECHNIQUES

This report examines the theory and evaluates

the results of over 200 tests of the use of low

frequency sonic interference techniques for the

detection of a human intruder in a confined area.

The conclusions are that this technique is potentially a significant improvement over conventional

methods with regard to area coverage and minimum

velocity detection.

This work examined the intruder signature and

background noise with respect to sonification frequency, source levels, intruder size, intruder

velocity, source types, area coverage (to 692 sq.

meters) , and geometric position of the source and

receiver in four radically different areas.

Interference effects of the intruder signature

and noise were analyzed with respect to bandwidth,

spectral content, and magnitude by both computerdrawn spectral displays, and specific frequency

corre lators

Key words : Human detector ; interferometric technique; low frequency acoustics.

1.0 INTRODUCTION

This report describes the results of a test program con

ducted in FY 19 73 to define the various parameters which

affect the probability of detecting an intruder by the

interferome tr ic measurement of his perturbation of an

acoustical sound field. This effort was funded by the

Atomic Energy Commission under the direction of Sandia

Laboratories

1 . 1 Background

The Electromagnetics Division of the National Bureau

of Standards has been active in the measurement and evaluation of interferometric techniques utilizing electromagnetic

as well as underwater acoustical energy for approximately
3 years. A single test was successfully conducted during

this period for determining feasibility of detecting an

intruder in a room using this principle. NBS personnel

presented the theory and background of this concept in

March 1972 for Sandia Laboratory personnel resulting in this

contract

.

1 . 2 Concept

The concept of the technique is as follows. A bounded

volume of air (closed room or building) is illuminated with

CW acoustical energy from a single speaker. As a result of

wall, ceiling, floor, and interior object reflections,

multipath transmission produces an acoustical field which,

in this report, will be referred to as a standing wave

pattern (SWP) . This pattern will be uniquely specified by

the phase and amplitude of pressure (relative to the source)

at all points within the field. The introduction of any

new object (scatterer) large enough to disturb the transmission paths will alter this standing wave pattern of the

entire volume. Any movement of the object will result in

a modulation of the standing wave pattern which is a complex

function of the object velocity, acoustical wavelength (A),

and angular direction of the object motion with respect to the

geometry of the source, reflectors, and the position of the

ob j ect

.

To illustrate a simple case involving only three

reflections the following analysis is presented (see

fig. 1.1).

With no intruder present, the resultant pressure wave

at the microphone is P^, which is the vectorial summation

of , aR d P3 (see fig. 1.1a). P^ will have a phase

and magnitude (relative to Pg) which is related to the

spreading loss, scattering loss, and acoustical wavelength.

With the introduction of the intruder an additional

pressure wave (P,p) (see fig. 1.1b) will be present whose

phase and magnitude also will be related to the above parameters

and, in addition, to the reflection coefficient (target scattering) at the aspect angle of the intruder. The resultant

(Pj) will have some new phase and magnitude different than P^,

which existed prior to the presence of the intruder. Therefore

2
VECTORIAL SIGNAL ADDITION

I a I.I b

P

s Source Pressure

P, Direct Path

P

2 Wall Reflection

P

3 Floor Reflection

P

T Target Reflection

P

, 1 P

*' P

T Figure 1.1
the intruder perturbation does not inherently require intruder

movement for detection. The principle is basically different

from Doppler techniques. In general, as the intruder traverses any path, the relative phase of ?^ will change with

respect to Pg through 360° at a rate which is proportional

to his speed (i.e. as the acoustical path length changes one

complete wavelength P,p will describe a complete 360° rotation

with respect to Pg) . The interference signal Pj will thus

contain both phase and amplitude modulation.

The frequency of this modulation is given by the following relationship:

P

_ intruder path length velocity

M acoustical wavelength '

where intruder path length velocity is the rate of change of

the acoustical path length due to the movement of the

intruder

.

In this simple case there is a direct relation between

the phase modulation in degrees and the amplitude modulation.

As an example:

Assume P

R

= 1.0 unit and P

T

= 0.035 units. The

amplitude modulation magnitude is ± 3.51. The

maximum phase displacement occurs when P^ is at

right angles with P^. This forms a right triangle

whose base is 1.0 and height is 0.035. The angle

formed is 2.0°. Hence 3.51 magnitude change is

equal to 2° phase change.

It is obvious that when all of the multiple transmission

paths contribute to the initial pressure wave P

R

, and multiple

paths to the intruder exist as well, the situation will produce a very complex modulation spectrum. The added effects

of appendage movements further increase the complexity.

An additional result of the multipath concept is that the

object is not required to produce a reflection for theoretical

detection. Even if the object is an absorber of acoustical

energy, the presence of the object interferes with some of

the multiple transmission paths, and this absorption in turn

produces a change in the standing wave pattern.
Any technique which measures these perturbations (either

or both phase and magnitude) can provide detection of the

intruder.

As with all measurements, the problem is to determine

the signal of interest in the presence of noise. In this

case noise (unwanted signal) arises from three distinct

sources

1.2.1 Ambient Acoustical Noise

Any acoustical energy present in the protected volume

containing frequency components near the source frequency

will produce interferences with the signals of interest.

N. = SPL A

, roT1, XTrr/P_. where P n is the sonification SPL and N. A AMBIENT R R A

is the relative noise produced by ambient acoustical energy.

The most obvious solution to reduce the effects of this interference is to choose an operating frequency at which the

background noise is minimal.

If this is impractical for other reasons, such as those

discussed in the following section, an absolute solution is

to increase the source levels such that the ambient energy

becomes a smaller contribution to the total received energy.

It should be noted at this point that given a detectable

signal, the detection sensitivity is not related to source

power, although the false-alarm rate may be expected to be.

The measured quantity for detection is the change of the

existing field pattern due to the introduction of the foreign

object. Therefore, the source acoustical power required for

an operational system is only related to the room volume and

reflecting properties and the requirement to mask any existing

acoustical energy present.

1.2.2 Noise Caused by Environmental Changes

Signal vector modulation can also result from changes

in the local environment. All changes such as air motion,

thermal gradient changes, temperature changes, wall

vibrations, and in addition, small animals (mice) can modulate the SWP . Each source of modulation will produce its

own effect, and the combination will produce a modulation

frequency spectrum.

5
This noise is assumed to increase in magnitude with

increasing source frequency. Additionally it is assumed

that the frequency of this noise is maximum at or near

zero Hz in the modulation spectrum, decreasing in magnitude

with increasing modulation frequency. This assumption was

made from the following hypothesis (for the purpose of this

report)

NEN

= AVP

R>

where is defined to be the normalized environmental noise

AP

R

is produced primarily by the relative change in

phase occurring in the vector summation of all signals

contributing to P

R

from all paths. As the source frequency

increases (shorter wavelength) , the environmental effects

will have a proportionately larger phase change for a given

environmental change. This would increase AP

R

, hence increasing N£N .

The modulation frequency of AP

R

will be determined by

the rate of change of those environmental effects which are

taking place. For example, a rapid window vibration will

produce a small change in the summation of P

R

, while dimensional changes of the building due to outside temperature

changes which affect the entire structure would be slow but

rather large.

With respect to small animals such as mice which might

be part of the environment (and which are a problem for

ultrasonic systems) , it was assumed that by using source

sonification of relatively low frequency (less than 10 kHz)

their scattering cross section would be made too small to

produce significant reflections.

1.2.3 Equipment Noise and Sensitivity

In order that the detection sensitivity be limited to

natural or background effects, the instrumentation used for

this evaluation was either designed or selected to insure

it did not become the limiting factor.

6
2.0 OBJECTIVE

The objective of this effort was to determine the interrelationship of the parameters which affect the detection of

an intruder. This work had the following goals:

1. Determination of the spectral distribution of the

intruder signal.

2. Determination of the spectral characteristic of

environmental noise as a function of enclosure

types

3. Analysis of the above for carrier frequency

opt imiz ation.

4. Examination of the effects upon detectab ility of

speaker-microphone physical position.

5. Measurements of area coverage by single units.

In order to accomplish these goals the following tasks

were identified:

1. Determination of instrumentation requirements.

a. Ambient noise analysis.

b. Spatial characteristics of standing wave

pattern

2. Construction and/or purchase of instrumentation.

3. Preliminary tests to determine intruder signature

as a function of carrier frequency.

4. Analysis of signal and noise for a variety of

conditions

5. Tests to determine volume coverage.

6. Final analysis and recommendations.

7
3.0 INSTRUMENTATION REQUIREMENTS

The purpose of this task was to provide the preliminary

instrumentation specifications. Decisions based upon this

work determined the selection of the discrete frequency to

be used for intrusion tests. It would additionally provide

receiver specifications. All references to sound pressure

levels given in dB use the standard reference of

0.0002 dynes/cm2 (hearing threshold). (Note: In some cases

a reference is included which equates specific dB levels to

environmental conditions. Generally a single frequency

sounds louder to humans than the same sound pressure level

in a broadband spectrum.) The following areas were evaluated

for this purpose.

3.0.1 Ambient Noise Characteristics

The spectral distribution of ambient noise should suggest

the most favorable carrier frequency ranges . The ambient

sound pressure level (SPL) magnitude would, additionally,

determine the minimum output acoustical power from the speaker

for given signal-to-noise ratios at specific carrier frequencies .

3.0.2 Target Scattering (reflection coefficient)

Rayleigh scattering theory predicts that the normalized

target scattering of a sphere is dependent upon carrier frequency when the ratio of circumference to wavelength is less

than 10 [1] •

By approximating the volume of an intruder as a sphere,

Rayleigh theory was used as one of the guides for determining

the lower test frequency.

3.0.3 Sound Pressure Level Field Patterns

The SPL field characteristic is important in determining

the dynamic range of the instrumentation required to measure

the signal levels encountered in a typical enclosed area.

8
3.1 Ambient Sound Pressure Level Measurements

As noted in section 1.2.1 the ultimate limiting factor

in determining the minimum acoustical power level required

for detection at a given carrier frequency is the ambient

sound pressure level. Preliminary ambient spectral measurements were performed in a variety of locations using a sound pressure level meter equipped with an active filter

The bandpass of this unit is ± 1/2 octave. This information

provided one of the guides for selecting the carrier frequencies used in testing detection characteristics.

The results of these tests are presented in figure 3.1.

With the exception of the Cameo Building, the noise power

slope was nominally 15 dB/decade from 100 Hz to 4 kHz. If

we assume that the intruder interference spectrum is directly

related to carrier frequency, we are justified in using a meter whose bandpass is a function of frequency. If this is

incorrect, one must convert the noise to a per Hz noise

power, thus increasing the slope of these curves. In Cameo,

a warehouse which contains no forced air ventilation or

machinery, the noise appears to level off at frequencies

below 1 kHz.

3 . 2 Target Scattering Calculations

No formal target scattering calculations were attempted,

due to the highly irregular shape of the human body. Rayleigh's

relationships of the effective acoustic scatter to geometric

cross section of a fixed rigid sphere were used as a guide

(see fig. 3.2) to determine the minimum lower test frequency.

The following assumptions were made:

a. Scattering is roughly omnidirectional.

b. The impedance mismatch of air to the human body

is large enough for the target to appear rigid.

c. The human body volume is nominally 0.07 to

0.085 meters 3.

d. The human body to be a sphere of that volume;

thus having a diameter of 0.51 to 0.55 meters.

9
10
NORMALIZED SCATTER FROM A RIGID SPHERE

10 i

1 1 1 1 1 1 1 1 1 1

2^0 . Circumference

X Wavelength a= Sphere Radius

a-- Scatter Cross Section

Figure 3.2

11
The circumference/wavelength ratio to be 3.0

minimum for a non-oscillatory target return

(fig. 3.2).

The wavelength thus required would be

, circumference tt n

_ n c _ . a = ^ = 3 0.51 = 0.53 meters corresponding

to a carrier frequency, f = 331 meters/sec/0 . 53

meter = 625 Hz.

3.3 Sound Pressure Level Field Pattern Measurements

Despite the fact that detection does not depend upon

absolute signal levels but rather upon the change of existing

levels, instrumentation requirements dictate signal levels

within finite values . Hence it is important to determine the

range of sound pressure levels to be expected for random

placement of the microphones.

A series of sound pressure field pattern measurements

were conducted in two rooms. In each case the room was

illuminated with acoustical energy at discrete carrier

frequencies. The room was first investigated for general

areas of low intensity. In all cases the fluctuations in

SPL occurred over short distances (0.3 meters). Over larger

volumes the general average appeared to be uniform. Two

patterns were investigated. In the first pattern, the

microphone was moved through a sixteen point grid in a vertical position. The microphone and speaker were separated

by a room divider. In the second case the SPL meter was

rotated in a horizontal circle (0.6 meter dia.). It is

obvious that the presence of the rotating meter will alter

the absolute SPL values. The purpose of these measurements

is only to determine the variation in SPL with position of

the sensing microphone. No individuals were in the enclosed

area during the readings.

The field patterns in figures 3.3 and 3.4 are representative of all field pattern measurements conducted. No measurements were made in the larger rooms (> 100 sq. meters)

because at the outset of the program, areas of that size

were not considered feasible. The vertical grid measurements

(fig. 3.3) were made over very small areas (10 cm sq.) and

indicate a general change of 10-14 dB with location. The

1 cm microphone represents 10% of the total grid dimension

and thus probably provides some averaging. The circular

patterns (fig. 3.4) were judged more appropriate in that

12
CP

13
CIRCULAR SOUND PRESSURE PATTERNS

Room 3064

2 kHz

Figure 3.4

14
there was a significant change as carrier frequency was increased. The circular path corresponded to 1.5 wavelengths

at 500 Hz and 6 wavelengths at 2 kHz. The number of maxima

and minima of the sound pressure, as well as the magnitude

of the change, increase with frequency. Measurements attempted at 4 kHz were unsuccessful due to signal amplitude

drift during the time interval required for the measurements

.

This type of drift is an example of environmental noise as

defined in section 1.2.

3 . 4 Conclus ions

The first parameter to be chosen was the carrier frequency which was to be used for the tests. The results of

the ambient noise tests favor the use of higher frequencies

(> 1 kHz) which would permit lower sonification levels for

given detection levels.

The results of the crude examination of target scattering

indicate a minimum test frequency in the vicinity of 500 Hz

due to the oscillation of effective scattering cross section

below 600 Hz. This minimum test frequency was left open

to change pending the experimental results

.

The results of the sound pressure field patterns indicate

considerable signal instability would be encountered at frequencies in excess of 4 to 5 kHz, since the signal strength

experienced considerable drift during the time period of

the tests.

Based upon the above, the test carrier frequencies were

chosen. It was concluded that the decade from 500 Hz to

5 kHz should be investigated. The following nominal frequencies were selected for investigation; 0.5, 1.0, 1.5,

2.2, 3.3, and 5.0 kHz. With the exception of 0.5 to 1.0 kHz

they were chosen to provide 1/2 octave steps. Hardware

constraints ultimately resulted in 1.5 and 3.3 kHz being

changed to 1.7 and 3.4 kHz.

On the basis of these measurements, the receiver sensitivity specification was chosen to insure that the

limiting factor in detection would be either the ambient

or the environmental noise, and not the receiver sensitivity

or noise characteristics

.

15
Because environmental noise could not be determined

until testing began, the ambient noise was initially chosen

as a guide in determining the sensitivity specification.

A subjective evaluation of various sonification levels

indicated that intensities exceeding 74 dB (1 ubar) were very

uncomfortable when produced at a single frequency. The

preliminary maximum sonification level was therefore chosen

to be 66 dB (0.4 ubar). The minimum measured ambient acoustical noise in test areas occurred in the auditorium at 4 kHz

(20 dB at 2800 Hz BW) . For the receiver to resolve this

level, the following analysis was used to determine minimum

receiver sensitivity:

20 dB = 0.002 ubar (2800 Hz BW)

= 0 .00004 ubar (1 Hz BW)

.

NA

=

( SPL AMBIENT

/ P

R)

x 10(n = " iTlLf^ X 100 = °'° n (1 Hz BW)

The receiver therefore should have a sensitivity adequate to

resolve a power level change equivalent to 0.011 of a 1 Hz BW

at a 66 dB sonification level.

At 1 kHz, in the instrument shop, the ambient noise was

46 dB (0.04 ubar) in a 700 Hz BW or .0015 ubar (1 Hz BW)

.

Therefore the sonification level would have to be greater

than

n SPL 0.0015 ubar , c ,„ fnn JTO P

R

= 7oT%= 0.01%= 15 ubar (97 dB)

for the receiver to influence the detectability at 1.0 kHz

carrier frequency.

The receiver total bandwidth was arbitrarily chosen to

be 1-75 Hz based primarily on the difficulties in providing

corrections to the phase servo at higher frequencies.

The dynamic range of the receiver was chosen by the

following analysis:

At 500 Hz, the ambient noise = 60 dB (0.2 ubar) (350 Hz BW)

.

The receiver BW = 75 Hz

Receiver is a peak detector.

16
Ambient noise in the receiver BW is

XT 0.2 ubar /2 n , 7 » N. = — — = 0.13 ubar

A /350/75

At a sonification level o£ 66 dB (0.4 ubar).

NA

= SPL^g/Pj^ = 0.13/0.4 = 0 .325.

The goal for the receiver dynamic range was therefore

set at ± 35% minimum. This specification assures that the

receiver does not saturate due to ambient noise.

Following the preliminary operational tests the receiver

response was changed from 1-75 Hz to 10-75 Hz due to unexpected target return levels in the 1-8 Hz range in room 3082.

This change reduced the dynamic range requirements, but did,

however, require frequency response corrections in all the

data analysis.

4.0 INSTRUMENTATION

The following section describes the instrumentation which

was either purchased or designed and constructed for use on

the program.

4 . 1 Sound Source

4.1.1 Transducer

The primary sound source used for these tests was a 50 watt, 4-way, ducted-port, commercial speaker system

(hereafter referred to as the speaker) . The unit was

capable of a 25-20,000 Hz response; however, the mid-range

speaker alone covered the test frequencies used for our

measurements. A second directional, exponential horn with

a 600 Hz low- frequency cutoff was used for comparison tests

(hereafter referred to as the horn)

.

Speaker input power is given for each test. The value

is calculated from E

2 /R (R = nominal speaker impedance)

.

No correction for power factor is included, so actual power

was probably somewhat less than that listed.

17
4.1.2 Amplifier and Oscillator

An NBS -designed battery powered audio amplifier was used

to drive the speaker. The battery operation permitted easy

movement and remote positioning without external cables.

A commercial variable -frequency audio oscillator was

used for the source oscillator and gain control.

4 . 2 Microphones and Preamplifiers

Three commercial omnidirectional condenser microphones

were purchased. Specifications are as follows:

Sensitivity -68 dB (re 0 dB = 1 V/uBar)

Frequency Response 30-15,000 Hz

Each microphone was provided with a broadband 46 dB

battery powered amplifier (see appendix) . Three additional

40 dB preamplifiers were available to provide extra gain when

low power tests were conducted.

4 . 3 Acoustical Hardware Mounting

The microphones were equipped with adjustable stands.

These stands were either taped to the walls or attached to

available structures. The speaker was placed on the floor

or a bench as selected for the tests

.

4.4 Receivers

The receiver functions by automatically adjusting the

phase and magnitude of the audio signal from the microphone

preamplifier for a null with respect to a hard-wired reference

signal from the source oscillator. The response time of

the servo loops is faster than any modulation frequency of

interest. By selecting a longer time delay for the servo

for either quantity (phase or magnitude) , the uncorrected

perturbations will be present from the detector (i.e. if

the phase servo is delayed to a response time of 1 Hz all

phase perturbations occurring at a rate greater than 1 Hz

will be present from the phase detector) . The receivers

are so equipped, with an optional time delay in both the

phase and magnitude control circuits.

18
The upper frequency response is controlled by a lowpass filter in the output amplifier. Thus, the output bandpass of the receiver is controlled by two filters. The receiver sensitivity to phase and magnitude changes is constant, regardless of changes in the received signal, because

the gain of that channel is continuously adjusted to match

the fixed reference.

4.4.1 Functional Analysis

The receiver functions are shown in figure 4.1 .

The receivers are composed of five plug-in printed

circuit boards mounted in a main frame which contains various

switches, meters, and connectors. The two input cards determine the frequency of operation, while the remaining three are

universal, covering the frequency range of 1 to 10 kHz. Five

meters are provided for monitoring the operation:

a. Signal Level Indicators (2) -- One each for reference and

signal channel.

b. Null Indicator -- Indicates the signal addition.

c. Amplitude Unbalance -- Indicates the amplitude unbalance

of the two signals.

d. Phase Unbalance -- Indicates phase deviation from 180°.

The reference and signal voltages are applied to the

input gain controls, which furnish manual gain adjustment.

The reference signal amplifier is followed by a manual phase

adjustment, achieved by a resolver which is continuously

adjustable through 360°. Appropriate resistor-capacitor

networks are furnished on each reference board for the specific frequency of operation. The resolver output is connected to a voltage variable phase shift network through a driver. This is followed by a tuned amplifier which reduces

noise and any harmonics present. The control circuitry for

the voltage variable phase shifter is selected for each carrier

frequency to insure uniform response of the phase control

servo. In the disable position of the phase lock switch,

the control is connected to an external connector for calibration purposes. In operation, this jack is grounded by a shorting cap. This feature allows manual adjustments prior

19
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to placing the servo in operation. As noted above, two

response times are available for the control circuitry.

In the fast mode, all perturbations up to 75 Hz are corrected

by the servo. In the slow mode, the time delay switch adds

an additional capacitor reducing the response to nominally

10 Hz C see response data in the appendix).

The signal channel is similar to the reference channel

except that a voltage variable attenuator replaces the phase

control circuitry, and the manual phase adjustment is

el iminated.

The reference and signal channel outputs are combined

in a resistive summing network. The output of this network

furnishes the input to the null amplifier. The null amplifier is additionally tuned to further reduce harmonics.

Automatic level control is obtained from a synchronous

detector whose inputs are the reference and null signals.

The output is a polarity reversible voltage, proportional to

the null magnitude. The polarity is determined by the phase

of the received signal relative to the reference. The detector output is filtered to remove the carrier frequency

components and connected through the lock switch to the

voltage variable attenuator control circuitry. The detector

output is further amplified and displayed on the amplitude

unbalance meter. The receiver is provided with dual outputs

from the amplifier, one direct, and the second through a 6 Hz

high-pass amplifier. Each output contains a 75 Hz low-pass

filter, thus limiting the receiver output to 75 Hz.

Automatic phase control is obtained from a similar

synchronous detector. The synchronous detector provides a polarity-reversible output voltage, proportional to the

phase displacement from 90° (for small phase angles). The

voltage is 0 VDC at 90° and 270°.

The reference and signal voltages are 180° out of phase

at null, so an additional 90° phase shift is required in

the phase detector reference voltage. This phase shift is

obtained from an automatic 90° phase shift card (quadrature

hybrid) . The automatic feature allows the units to operate

over a wide frequency range while maintaining the 90° phase

displacement

.

The automatic 90° phase shift is accomplished by

servoing a voltage variable phase shift network to 90° by

use of a synchronous detector. All of this circuitry is

contained on the printed circuit card.

21
The post-detector portion of the phase detector card

is identical to the amplitude detector card with the exception

of the absolute gain. Gain of the amplifiers is adjusted

to provide equal output signal from either the phase or

magnitude for a given input vector addition. This requirement was noted in section 1.2.

The following steps are followed to place the receiver

in operation.

a. Time delays out.

b. Lock switch out.

c. Adjust reference level to 3 VRMS

.

d. Adjust signal level to 3 VRMS.

e. Adjust resolver for null on null indicator.

f. Lock switches to lock.

g. Time delay to delay for parameter of interest.

The high-pass amplifiers are provided to allow additional

signal compression if the low frequency components of the

noise below 6 Hz are large with respect to the higher frequencies. The magnetic recorder used in these tests had a dynamic range limited to 40 dB . If the range of signal

magnitude exceeded the recorder capabilities, the higher

frequency information would be lost without this provision.

This feature was not required for the tests.

Six receivers were constructed. As noted, the receiver

frequency is determined by the input printed circuit cards.

Three sets of input cards at 2.2 and 3.4 kHz were available

for simultaneous triple tests. In addition, one set at

1.0, 1.7, and 5.0 kHz were used for single tests. A set

at 500 Hz was fabricated, but we experienced problems with

the voltage variable phase shifter. Early intruder tests

indicated that 1.0 to 5.0 kHz covered the frequencies of

interest, so no further effort was expended on this unit.

Measured receiver specifications:

Dynamic tracking range

Phase ± 50°

Amplitude (nominally ± 10 dB)

22
Dynamic range

Phase ± 20°

Amplitude ± 351

Bandwidth 75 Hz

Minimum detectable change (1 Hz BW)

0 .01%

0.07°

4 . 5 Recording Capabilities and Signature Analysis

All tests were monitored in real time with a dual channel

high speed strip chart recorder. In addition, the outputs were

magnetically recorded on a four-channel FM tape recorder.

Following each test, the recording was replayed on the strip

chart recorder to insure that an accurate recording had been

obtained. Variable active filters were available during the

tests to examine portions of the frequency spectrum when

required.

The primary data analysis for spectral examination of

both the noise and the intruder signature for this program

was accomplished by fast Fourier transform calculations with

the computer. The analog data from the magnetic tape was

first digitized, and then individual power spectral density

plots were computed. These were then integrated into a three

dimensional presentation (see fig. 6.1) which displays power

vs. time and frequency.

Time is plotted from left to right, frequency from back

to front at .997 Hz per interval, and log amplitude of the

signal is plotted vertically. These displays enable a rapid

evaluation of the spectral content of both noise and of the

intruder disturbance. A short section of the noise "floor"

is visible on the right side of each plot and may be used to

estimate the signal-to-noise ratio of each test. A better

estimate may be obtained from the correlation plot.

After the completion of the computer spectral analysis,

the primary analysis technique used was single frequency

correlation. The correlators used for this work consist of

an internal two-phase oscillator, analogue multipliers, and

filters. The correlator output is a slowly varying dc voltage

proportional to the total signal -plus -noise power in a 0.2 Hz

band centered at the oscillator frequency. Five correlation

frequencies, 2, 4, 10, and 15 Hz, were available for this

analysis. These units provided an additional benefit in that

they could be used in real time to monitor the intrusion.

23
For all displays showing

the correlators was compensate

response of the receivers.

correlated data,

d to correct for

the gain of

the frequency

5.0 TEST LOCATION DESCRIPTIONS

The following is a description of the test areas used for

the intruder perturbation analysis. The intruder path is

shown in each figure. Microphone position and speaker locations are shown in the figures. Exceptions will be noted

separately. The term "geometry" is used to indicate relative

microphone and speaker location throughout this report.

5.1 Room 3082, Radio Building

See figure 5.1. This 83 sq. meter (890 sq. ft.) laboratory was chosen for the initial test. It is slightly larger

than what is normally considered to be the effective coverage

of ultrasonic Doppler units (60 sq. meter). The ceiling and

walls are copper foil over gypsum board with a concrete floor.

Ceiling height is 3 meters. The room contains no windows but

has a forced air ventilation system which ran continuously

during the tests. Considerable air is flowing at all times.

There are several room dividers and benches. The ducts for

the ventilation system were suspended from the ceiling, and

extended downward 0.75 meter, to produce a highly irregular

volume

.

5 .2 NBS Auditorium

See figure 5.2. The NBS auditorium is a conventional

auditorium of 345 sq. meters (3705 sq. ft.). The walls and

floor are concrete, and the only obstructions are 534 seats

and the stage. The walls are covered with wood paneling.

The equipment was set up in the projection booth so the

intruder action could be observed. The ceiling is approximately 6 meters high, thus producing a rather large volume.

Ventilation is provided by forced air through overhead vents.

Considerable air movement was present during the tests.

24

NBS Auditorium

Figure 5.2

INTRUDER PATH

SOUND SOURCE

MICROPHONE
5 . 3 NBS Instrument Shop

See figure 5.3. This shop is located in wing 3 of the

main Radio Building. It contains a floor space of 600 sq.

meters (6456 sq. ft.). The length is about three times the

width. It is very crowded with large machinery (see figure

5.3 for placement). The outside wall has windows for the

entire length, from approximately 1 meter above the floor up

to the ceiling. The ceiling is at two levels, divided by

the center line of the length and is formed of concrete beams.

The outside half is about 3.5 meters high, while the inside

section is 5.5 meters. Several large double doors result

in rather unstable reflecting surfaces. Heating is provided

by baseboard steam heat. The speaker location, shown midway

in the plan drawing, is a tool crib with large counters.

The equipment was set up in room 3332 at one end. This room

has windows so that the intruder could be viewed during his

trave rs e

.

5 . 4 Cameo Building #5

See figure 5.4. The Cameo Building is a frame warehouse

with corregated steel roof used to store surplus equipment.

The room has a floor space of 487 sq. meters (5241 sq. ft.)

and a distance of 12 meters from the floor to the base of

the roof. The room is full of shelves and racks. A large

steel roll-up door was of very loose fit. Chirping birds

were present inside and were flying during the first part

of the tests. Forced air overhead heaters were present,

but it is not known if they operated. Outside temperatures

were in the range of 50-60°.

6.0 PRELIMINARY EVALUATION, ROOM 3082

SPECTRAL DISTRIBUTION OF INTRUDER SIGNATURE AND NOISE

This initial intruder test series was designed to investigate the spectral characteristics of both the background noise

and the intruder signature. (In this report background noise

refers to all noise from the receiver.) The results also

provided a basis for the determination of target strength

(intruder scattering)

.

27
28
SOUND SOURCE |

MICROPHONE

INTRUDER PATH

r

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Cameo Building

Figure 5.4

29
6.1 Test Description

These tests were performed in Room 3082 (fig. 5.1). The

room was chosen because it was somewhat larger in size than

the typical Doppler ultrasonic area of coverage for a single

unit. Three microphones were placed as shown, and the intruder path was selected to provide some detection from each

unit. No pretesting of locations was conducted. All test

equipment was set up outside in the hallway. In each test,

the intruder opened the door, stepped in, and closed the door,

remaining motionless for 5 seconds before proceeding along

the path. The intruder again halted for 5 seconds before

exiting the room. The intruder path was practiced for several

runs before the tests to insure as nearly identical intrusions

as possible. Electrical power to the speaker was 22 mw, and

typical sound pressure levels ranged from 0.07 to 0.2 ubars.

This level is typical of a business office.

Tests were conducted at the following five carrier frequencies: 1.0, 1.7, 2.2, 3.4 and 5.0 kHz. Originally 500 Hz

was to have been included, but difficulties were experienced

in producing exact calibration, so preliminary tests were

conducted without that frequency.

A minimum of one test from each microphone at each

carrier frequency was conducted. In addition, several tests

were performed to insure repeatability from test to test.

Also, three tests using the output jack from the sound

pressure level meter in place of the microphone -preamplifier

were conducted to determine if any unanticipated effects

from the microphone might be encountered. None were.

6 . 2 Test Results and Analysis

The spectral displays of the intruder interference from

each position at each carrier frequency (except 5.0 kHz) are

presented in figure 6.1. The results from the tests at

5.0 kHz were very poor because of considerable phase and

amplitude fluctuations during the tests. This condition was

noted as a potential problem in section 3.0 in discussions

concerning the field patterns . The receivers did not have

sufficient dynamic range and made operator corrections

necessary during the tests at 5.0 kHz.

30
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SPECTRAL DISPLAYS INTRUDER SIGNATURE VS CARRIER FREQUENCY

Room 3082

Figure 6.1
As noted in the instrumentation section, the presentations display time from left to right, perturbation frequency

from back to front, and log amplitude of the signal in the

vertical direction. In addition, the 10 Hz low-pass response

of the receivers reduces the output by 17 dB at 1 Hz, thus

reducing the low frequency magnitude. At a later date, a correction was added to the computer program which produced

displays of absolute response. See figure 6.2 for a comparison of this effect.

The intruder entry and exit in which the door was opened

(including the additional acoustical energy from the closing)

is quite evident in all displays. In particular, position 2,

which was next to the door, resulted in very large impulse

energy during the door closure. This should be ignored when

analyzing the data for intruder signatures.

The most remarkable result of these tests was the wide

area of coverage. Detection was literally 100% from any

position is the room during the entire time the intruder was

moving in the room. It should be remembered that this room

is 1.5 times the size normally covered by an ultrasonic unit.

The second unexpected effect was the exceptional sensitivity of position 3, located on the far side of the room

on top of a file cabinet 2 meters high. Position three

appears superior to number one at all frequencies. Position

three is nearly as good as position two which was centrally

located, and thus being, on the average, closest to the

intruder

.

The spectral distribution is generally broadband, with

increasing slope as the carrier frequency decreases, thus

supporting the hypothesis that the intruder signature

spectrum is roughly proportional to carrier frequency.

Since the general character of the signature was broadband, the same data was analyzed with the correlators (see

section 4.5) to determine if they could provide an effective

substitute for the spectral displays. Spectral displays are

quite expensive (nominally $35 each). In addition, correlation is available either in real time or immediately after

the test was conducted. Computer analysis required the services of extra personnel and several days of elapsed time.

The correlations are presented in figure 6.3.

32
POSITION I -22mW

ER

A
f
RECEIVER OUTPUT VS. CORRELATION

15 Hz CORRELATION

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Figure 6.3

34
33
It is apparent that correlation provides an adequate

method of investigating the signatures and noise of all the

tests. In fact, signal to noise ratio may be more easily

determined from the correlations than from the spectral displays. This conclusion was confirmed by making the same

comparison with tests conducted in the shop and auditorium.

An evaluation of the background noise supports the

assumptions of section 1.2 describing the spectral composition of the noise. See the right-hand side of the displays

in figures 6.1 and 6.2.

Note: If the limiting noise is ambient noise, the

spectrum will be flat over the very narrow bandwidth

of the receiver. If the limiting background noise is

produced from environmental effects, the lower frequencies contain more power (see section 1.2.2).

During the tests, the ambient SPL was 14 dB at 1.0 kHz

and 0 dB at 3.4 kHz (1 Hz BW) . The 3.4 kHz spectral displays

indicate a low frequency (< 3 Hz) response of about +10 to

+15 dB with respect to the general spectrum response, thus

indicating environmental effects. The noise spectrum at

1.0 kHz is nearly flat, indicating that any environmental

effects have been swamped by the additional 14 dB ambient

noise. The relative effect of ambient as opposed to environmental effects is examined further in later tests.

Both the spectral displays and the correlations indicate

superior signal to noise ratio at the two higher frequencies.

In order to determine if this was due completely to increased

ambient noise at the lower frequencies, an analysis of the

intruder signature magnitude was performed.

Intruder signature magnitude as a function of perturbation frequency with a 201 BW was measured for each carrier

frequency and position.

Note : Throughout the remainder of this report the

intruder signature magnitude is referred to as target

strength. This value is the maximum ASPL produced

by the presence of the intruder. Because the intruder signature is broadband, this parameter is listed

for a specified bandwidth.

These data were corrected for receiver response and for the

difference in microphone SPL to average room SPL (the receiver

output is A.SPL/SPL) . The received SPL at the microphone will

normally be different than the room average. All values

from each carrier frequency were averaged, and the response

35
for each carrier frequency plotted in figure 6.4. Due to

the corrections required, and the variations in SPL with

which the intruder interfers as he traverses the room, this

is not a precise measurement. It does, however, produce

a general average with which to judge the validity of the

assumptions concerning the effects of carrier frequency upon

target strength.

The results of these measurements produced a peak of

6% modulation for carrier frequencies from 1.7 kHz to

5.0 kHz. This corresponds to a ASPL/SPL of -25 dB . The

response at 1.0 kHz is nearly - 5 dB with respect to the other

frequencies. This loss of target return is attributed to

a loss of effective scattering cross section as the acoustical

wavelength approaches the dimensions of the target (see

section 3.3). The cause of the increased target return

at 0.2 5 Hz for 1.7 and 3.4 kHz while both 2.2 and 1.0 kHz

drop is unknown at this point. It is suspected to be related to the particular intruder velocity and wavelength

since similar response occurs at nominal harmonics.

Note: The target strength value at 0.25 Hz required

a receiver correction factor of +27 dB. This, in

addition to the problems of recording and analyzing

these low frequencies, reduces our confidence in this

data point

.

The assumed spectral dependence of the signature upon

carrier frequency was shown in both the spectral and correlation displays (figs. 6.1 and 6.2). Very little high

frequency response (except for the door operation) in

indicated at 1.0 kHz, while all microphone positions show

increasing high frequency response with increasing carrier

frequency

.

Note: Spectral plots display log power in 1 Hz bandwidth while target strength is plotted as voltage in

20% BW so direct comparisons cannot be made.

6 . 3 Conclusions

The goals of this test series was satisfactorily

achieved. The intruder signature was identified as broadband, peaking at relatively low frequencies (1-8 Hz)

.

The intruder signature frequency response is roughly proportional to carrier frequency.

36
co ID CvJ 8*O

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37
The noise spectrum agreed completely with the hypotheses

stated in section 1.2 The environmentally produced noise

spectrum is highest at very low frequencies and decreases with

increasing frequency. If the ambient noise becomes the

limiting factor, the receiver output noise spectrum is flat

over the range of interest.

The validity of using a correlator as a general data

analysis tool was confirmed. This allowed a larger number

of tests for a given time and fund restriction.

The most significant result of the test series was the

absolute detection from all microphone positions throughout

the entire room. All subsequent tests were planned for larger

volumes to enable a more detailed comparison.

The analysis of the intruder target strength indicated

that the 500 Hz tests were not required. Target return at

1.0 kHz was nearly -5 dB with respect to the other four frequencies. Adding to the reduced target return is the increased ambient noise which further reduced the signal to

noise ratio.

7.0 INTRUDER TESTS, NBS AUDITORIUM

A total of 67 intrusion tests were conducted at five

carrier frequencies on five separate occasions. These tests

examined the spectral content of both the intruder signature

and the noise, with respect to carrier frequency. Ambient

and environmental noise were examined in detail for their

individual effects. In addition, the effects of intruder

velocity, sonification source, geometry, intruder size, and

multiple intruder actions were investigated.

7 . 1 Test Description

All tests which are analyzed in this section were conducted in the NBS Auditorium. The microphones and speakers

were placed as shown in figure 5.2. Microphones one and three

were taped to the walls 2 meters from the floor, while microphone two was taped to the back of a seat with the sensing

portion approximately 15 cm above the seat back. Two speaker

positions were used; in geometry A the sound source was placed

upon the seat arms while in geometry B the sound source was

set on a box about 2 meters above the stage floor. No preselection of speakers or microphones was used for any test.

38
The instrumentation was set up outside the auditorium

for the first test series. It was placed in the projection

booth in the remaining test series so that the intruder

action could be observed.

Unless noted differently, each intrusion consisted of

the following: the intruder entered the right auditorium

door, waited 5 seconds, walked down the right aisle, paused

5 seconds at the stage, walked across the front, paused again

for 5 seconds, walked up the left aisle, waited 5 seconds

and then exited.

Two intruders were used: intruder A, weighing 70 kgm,

and intruder B, weighing 100 kgm. Intruder velocity was

nominally 0.9 m/sec.

This test area provided a nearly unobstructed volume which

would, in theory, produce a minimum of standing wave fluctuations due to the lack of reflecting surfaces. It is extremely

large in volume due to the high ceiling. The room also provided an area with few non-rigid surfaces such as windows and

doors. The ability to conduct a large number of tests at

3.4 kHz can be attributed to the latter feature.

7 . 2 Test Results and Analysis

The various types of tests are separated into indi

vidually headed sections.

7.2.1 Intruder Signature Analysis

The intruder signature spectrum was analyzed as a function of carrier frequency to determine if the spectral

character exhibited in room 3082 would be confirmed in an

entirely different test area. Several intrusions were conducted at 1.0, 1.7, 2.2 and 3.4 kHz. Tests at 5.0 kHz were

attempted on several occasions but signal stability was poor,

and the results were not repeatable.

A spectral analysis of the signature from position 2,

geometry B, of these tests is presented in figure 7.1. The

signature frequency response shows a clear carrier frequency

dependence. In geometry B the intruder passes directly between the speaker and position 2 when passing in front of

the stage, so the two aisle traverses are most representative

39
I
of the response. At 3.4 kHz the response shows very little

decrease for the four correlated frequencies (2, 4, 10, 15 Hz),

while at 1.0 kHz no response is indicated at 10 or 15 Hz.

The response at the two intermediate carrier frequencies is

approximately proportional to the carrier frequency ratio.

An examination of the noise from these tests clearly

supports the hypothesis presented in section 1.2 and the

results from room 3082. At 1.0 kHz, the noise level is

nearly the same at all correlated frequencies indicating

that ambient noise is the predominant limiting noise source.

At 3.4 kHz, the noise at 2 Hz is approximately 3 dB greater

than the 2 Hz noise at 1.0 kHz. At 15 Hz, the noise from

3.4 kHz is at least 3 dB less than that observed at 1.0 kHz.

The higher noise at the lower frequencies implies that the

environmental effects produced the noise.

7.2.2 Sonification Level vs. Noise and Intruder Signature

Magnitudes

These tests were conducted to verify that the effect of

ambient noise could be reduced while that produced by environmental effects would be unaffected by increasing the source

level. A second purpose was to establish the independence

of intruder magnitude with respect to source levels.

The results of the tests are shown in figure 7.2.

Three source levels were used for the tests at 3.4 kHz.

The results at 3.4 kHz are used to provide environmental

noise levels sufficient to demonstrate the principle.

Very little increase in noise level is evidenced when

reducing the input electrical speaker power from 60 to 15 mw

Independent measurement of the noise indicated nominally 2 dB

change at 4 Hz with no change at all at 2 Hz. When the power

was further reduced to 9 mw , a significant increase in the

noise is encountered at all correlated frequencies. The

results of the environmental noise are still evident at

2 Hz indicating that the ambient noise has not become the

total limiting source of noise. The correlator output is

the summation of all signal energy present.

An independent noise analysis was also performed to

determine the agreement of measured and calculated noise

levels. Tests were conducted at 2.2 kHz with four power

levels. The results are as follows:

41
INTRUDER SIGNATURE

SPECTRAL DEPENDENCE VS CARRIER FREQUENCY

1.0 kHz 1.7 kHz 2.2 kHz 34 kHz

Receiver Output

\ Ml 1 V 1/

- I

| Star! pause pause Slop|

mmm m

i

NBS Auditoriu

Figure 7.1

;4o

60mW Speaker

Position 2 - Geometry ( Intruder A -70 kg

I Sec/Division
)
8 <e °o:
Speaker

Indicated Noise (1 Hz BW)

From the Receiver

Input Power SPL 10 Hz 2 Hz

960 mW 70 dB 40 mV PP 200 mV PP

240 65 50 200

60 58 100 240

15 51 175 280

Note: The receiver output sensitivity is 1 volt

for each 3.51 change in the ratio of ASPL/SPL,

which in this case is N^. Thus , for constant

ambient noise, the indicated noise will decrease

as the SPL produced by the source is increased.

Ambient noise level was 29 dB or 0.0055 ubar in a 1400 Hz BW (0.00015 ubar in 1 Hz BW) . At 15 mW the sonification SPL was 51 dB or 0.07 ubar. From section 1.2.1,

NA

= SPLAMB /SPL SONIFICATION

= 0-00015/0.07 x 100 = 0.21%.

The receiver-indicated noise was 175 mV PP , or 62 mV RMS.

The receiver sensitivity is 3.5%/V. Indicated noise is

therefore 3.51/V x .062 = 0.22%

No attempt to justify the close agreement is offered,

due to the multiple corrections and assumptions required to

perform this calculation. These include the record levels

,

receiver calibration, and the assumption for calculation

purposes that the measured ambient noise was flat over the

bandwidth. If the calculations are compared at 60 mW there

is an error of about 3 dB

.

The noise at 10 Hz dropped approximately 6 dB for each

6 dB increase in power, except for the final increase. This

indicates that environmental effects have become predominant

at 70 dB. At 2 Hz there is very little noise level change

with increasing power, indicating that the environmental

effects are predominant.

The intruder signature magnitude shows negligible

changes at each of the three source levels considering

that they are the results of three separate intrusions. As

expected, the 9 mW signature magnitude is somewhat higher,

again due to the correlator indicating the summation of both

signal and noise.

43
Intruder signature is again shown as a function of source

level in figure 7.3 with the horn as the source in these

tests. Two input power levels are shown, and no change in

the intruder signature is demonstrated. The noise level did

not change, indicating that environmental noise is predominant

at these levels.

7.2.3 Intruder Signature as a Function of Source

and Receiver Characteristics

The absence of change in the intruder signature with

respect to the sonification source and the type of microphone

preamplifier are shown in figures 7.4 and 7.5.

A comparison of the intruder signature and noise when

the sound level meter was substituted for the microphone and

preamplifier is shown in figure 7.4. The sound level meter

was substituted for the microphone and preamplifier and the

recorder output was used as the receiver input. Duplicate

intrusions were conducted. No significant change in the

signature is indicated, while the noise actually increased

slightly. The increase was not evidenced in other comparisons

and is attributed to a change in the environment for that

test

.

The results of the direct substitution of the exponential

horn for the speaker are shown in figure 7.5. The horn

consistently produced higher sonification levels for equal

electrical input, so the input power was adjusted for nominally

the same acoustical levels as measured with the sound pressure

level meter. Some differences were expected from the directional effects of the horn, but little is indicated.

The computer-produced spectral display does indicate

a higher 1 Hz peak from the horn at the instant the intruder

passed directly in front of the source.

This lack of change in signature when the directivity of

the source is changed radically is attributed to the multiple

reflections producing an extremely complex standing wave

pattern. The standing wave pattern probably becomes somewhat

independent (in average magnitude and complexity) of the

source characteristics when examined at points remote from

the source position.

44
SONIFICATION VS. NOISE AND INTRUDER SIGNATURE

15 mW 60 mW

ENTER EXIT 3.4 kHz Horn Geometry B , Position 2

Intruder B NBS Auditorium

Figure 7.3

45
INTRUDER SIGNATURE

MICROPHONE PRE AMP VS. SOUND LEVEL METER

MICROPHONE PRE AMP SOUND LEVEL METER

tsTAR ENTER

STOP!

EXIT

20 dB

2.2 kHz, 60 mW Speaker

Geometry A, Position 2

Intruder A

NBS Auditorium

Figure 7.4

46
INTRUDER SIGNATURE - SPEAKER VS. HORN

3.4 kHz , Intruder B

Geometry B, Position 2

NBS Auditorium

Figure 7.5

4 7
7.2.4 Detection as a Function of Geometry

The effects of the microphone and speaker placement

are shown in figure 7.6. In each of the tests, the results

from these microphone positions are compared. Three tests

are shown: in the first two a common geometry was used and

two frequencies were compared; while in tests two and three,

the effect of changing the source from geometry A to B is

compared.

In each test, the results from position two were nearly

symmetrical. This was expected, since the geometry is

symmetrical with the room. In positions one and three, the

maximum detection occurred when the intruder was traversing

the aisle closest to the respective microphone. The difference in detection from one side to the other for a given

intrusion was not large, nominally 3-4 dB.

There appears to be a small detection improvement using

geometry A, which can be attributed to the fact that the

auditorium acoustics are designed for minimum reflections

when the sound source is in front. The reduced number of

reflections should reduce the standing wave complexity, and

hence, the intruder signature. Most tests were conducted

in geometry B so as not to take advantage of this fact.

7.2.5 Spectral Distribution vs. Intruder Velocity

Tests were conducted to determine if there is a direct

dependence of the intruder spectrum upon the intruder velocity.

The results of one of these tests are shown in figure 7.7.

The intruder velocity was reduced to 0.4 meters/sec and

compared directly with a standard intrusion of 0.9 meters/sec.

At the reduced velocity, the response at 10 Hz was nearly

nonexistent. The response at 2 Hz was approximately the

same, and detection occurred for the entire intrusion. The

response at 4 Hz was slightly reduced.

7.2.6 Intruder Size Comparison

The effect of the intruder size upon effective target

return was investigated in 6 tests. The results of two of

these tests are shown in figure 7.8.

48
DETECTION VS. GEOMETRY

2.2 kHz Geometry A 1.7 kHz Geometry A 1,7 kHz GeometryJi

Position 2 , Intruder A

NBS Auditorium

Figure 7.6

49

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51
In each, the intruder conducted two intrusions to test

repeatability. The repeatability was very good at 10 Hz

for all comparisons. Some small variations in the 2 Hz

response occurred.

At 10 Hz, there appears to be about a 3 dB increase in

target return from the large intruder. There is a definite

increase in target return at 2 Hz also; however, it does not

appear to he as large. Integration of all the energy under

the curve would be required to provide a precise answer.

7.2.7 Additional Exploratory Tests

In addition to the tests required to satisfy the

goals of the program, several additional tests of a feasibility nature were conducted.

Four tests were conducted in which a radio was turned

on and the spectral content of the receiver output examined.

The radio was -6 dB with respect to the sonification level

(broadband) as measured with the sound pressure level meter.

The effects of one test are shown in figure 7.9. The

intruder entered the auditorium, walked to the front, and

turned the radio on and then exited. After 110 seconds he

returned, shut the radio off, and again exited.

The purpose of the test was twofold. If exterior

broadband noise were coupled into the protected area, it

could produce false alarms. The results of this test, however, indicate a minimal increase in noise level and

certainly not of the same character as that of an intruder.

The second purpose was to investigate the possibility

of masking the sonification energy for producing a nominally

covert system. Even in this test, in which the radio spectrum

was not filtered at the source frequency, it appears likely

that a system could be produced in which the sonification

energy would go unnoticed by a casual intruder. That is,

if desired, a radio could be used to prevent the intruder

from being aware of the alarm system.

A second test is shown in figure 7.10 in which the

intruder performed a random pattern of movements. The horn

was used for a sonification source. The intruder action

is noted and will not be discussed except to comment on the

52
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section in which the intruder was crawling between the seats.

Although the signal to noise ratio during that part of the

test was low (3-4 dB) , the equipment was not designed to

detect low frequency modulation which is a characteristic of

slow movement. A low frequency correlator would be desirable

to take advantage of the reduced velocity.

In addition to these tests, an experiment was conducted

in which an arm was extended through the projection booth window

and waved. No effect was observed at 1.7 kHz, minimal at

2.2 KHz, but was quite noticeable at 3.4 kHz. The

susceptibility to small object movement indicates that for

any environment with small animals such as mice, rats, or

cats, lower operating frequencies would be more desirable.

7.3 Conclusions

The results of these tests provided data which supported

all of the initial concept described in section 1.2 and the

analysis. The spectral dependence of the intruder signature

agreed completely with the measurements from room 3082. However, no loss of target strength at 1.0 kHz was found in the

auditorium as had been the case in room 3082. No tests were

conducted at 500 Hz, which was selected in section 3.2 to be

the probable lower limit for uniform target return. Target

return magnitude measurements are rather difficult to perform

because the receiver measures ASPL of the standing wave pattern. In this case, P is the SPL measured at the microphone

position. It is unlikely that this value will equal the SPL

average for the room. A correction for this difference must

be added when calculating absolute target return values.

The accuracy of the average room SPL measurements is estimated

to be no better than 3 dB.

Reliable receiver operation at 3.4 kHz occurred in the

auditorium. This is attributed to a solid and unobstructed

room, where environmentally produced signal changes were

relatively small.

The results of the noise tests indicate that the factor

limiting detection will be environmental noise, even with

reasonably low sonification pressure levels. No significant

increase in noise was detected in the auditorium at 3.4 kHz

until the electrical power to the speaker was reduced to

15 mW, which produced a SPL of about 54 dB.

55
The analysis at 2.2 kHz also indicates that sonification

levels of 60-66 dB provide adequate power to mask ambient

conditions

.

8.0 INTRUDER TESTS, NBS INSTRUMENT SHOP

Four test series were conducted in the instrument shop

consisting of 57 intrusions. The general parameters examined

were the same as for those tests conducted in the auditorium

[section 7.0).

8 . 1 Test Descriptions

These tests were conducted to determine the effects of a very complex environment. This room was considered likely to

be a difficult area to protect, because its 3:1 length to

width ratio and, because of the presence of a number of very

sizable machines. In addition, the entire length of outside

wall consists of windows, which provides a flexible surface.

Three large double doors are also subject to vibration and

provide little attenuation path to the noise occurring in

the hallway. The instrumentation was set up in a small

room at the far end of the shop area. The small room is

enclosed with windows, thus allowing the operator to view

the area.

The microphones and speakers were placed as shown in

figure 5.3. Three geometries were used; in A the microphones

were placed on the center posts while in B and C they were

attached to the outside wall 3 meters from the floor. In

geometry A and B, the speaker was placed on a cabinet 1.5

meters from the floor, but it was moved to the far end for

geometry C. No preselection of speakers and microphones

was used prior to the tests.

In each test the intruder exited the small room, waited

5 seconds, began the traverse as shown, pausing 5 seconds

prior to reentering the room. The intruder made a complete

trip from one end of the shop to the other.

The same two intruders were used, A at 70 kgm and

B at 100 kgm.

56
8.2 Test Results and Analysis

The following sections describing the test results in

the instrument shop are divided into sections containing

similar tests.

8.2.1 Intruder Signature Analysis

The intruder signature spectrum was again analyzed as a function of carrier frequency in geometry B, position 1.

The results of these tests are shown in figure 8.1. which

includes both computer spectral displays and individual correlated frequencies. Tests at 5.0 kHz were attempted on

several occasions but none were even completed, due to signal

instability. The tests at 3.4 kHz were also difficult to

perform for the same reason. The effects of the environment

in this area were much more pronounced than those encountered

in the auditorium.

The results of the spectral analysis support the previous

results of both the auditorium and room 3082. The response

is nearly proportional to the carrier frequency. As in the

auditorium, there is no clear evidence of target return loss

at 1.0 kHz. However, a comparison made in geometry A (fig.

8.2) indicates a loss of about 3 dB . As noted in section 7.0,

the measurement is difficult to perform. An added problem

is evident from the oscillatory nature of the response as

shown in figure 3.2. Small changes in the circumference/

wavelength will produce changes of 3 dB in the effective

scatter.

The background noise is very similar to that encountered

in the auditorium with the exception of an increase in environmental noise at 3.4 kHz which would be expected from the

large window area. Ambient noise was measured at microphone

position #1 and found to be +26 dB at 3.4 kHz, somewhat

lower than the general results in section 3, but very near

that of the auditorium. Noise at microphone position #2

was 34 dB and at position #3 was 37 dB.

8.2.2 Sonification Level vs. Noise and Intruder

Signature Magnitude

Tests were again conducted to verify that the effects

of ambient noise could be reduced by increasing the sonification level. The results of these tests are shown in

\

57

TARGET STRENGTH AS A FUNCTION OF FREQUENCY

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Figure 8.2

59
INTRUDER SIGNATURE SPECTRAL DEPENDENCE VS. CARRIER FREQUENCYJ
TARGET STRENGTH AS A FUNCTION OF FREQUENCY
figure 8.3a. The power level was increased 6 dB with, no

apparent change in the target strength. An independent

measurement of the noise power for these two tests produced

the following results.

Speaker

Input Power SPL 10 Hz 2 Hz

240 mW 60 dB 80 mV PP 200 mV PP

60 53 150 320

The ambient noise at position 1 was 26 dB or 0.005 ubar.

Converted to a 1 Hz BW it is 0.00013 ubar. At 60 mW the SPL

was 53 dB or 0.09 ubar. From section 1.2.1,

Noise = NA

= SPLAMBIENT/ p R

= 0.00013/0.09 x 100 = .14*.

The receiver-indicated noise is 150 mV PP , or 53 mV RMS.

The receiver sensitivity is 3.5%/V. Indicated noise is

therefore 3.5%/V x 53 mV = 0.181; a variation of only 2 dB.

A comparison of the 10 Hz noise levels shows a drop of about

6 dB for a 6 dB power increase while at 2 Hz the noise decreased 3 dB. This indicates significant environmental noise

present at the lower frequencies. Again, the intruder signature magnitude remained the same for either power level.

8.2.3 Intruder Signature as a Function of Source Characteristics

A comparison was again made of the effects upon detection produced by changing the sonification source. In

figure 8.3b, the horn was replaced with the speaker. When

comparing the 2 Hz response of the intruder for the two sources (fig. 8.3a to fig. 8.3b), some differences appear.

The signature produced with the horn has two high points

which occurred as the intruder passed the source position.

His route brought him past the source twice. The directionality of the horn would be expected to produce this type

of reaction because a greater portion of the energy is

transmitted directly in front of the horn. No other significant differences were observed.

60
INTRUDER SIGNATURE

AS A FUNCTION OF SOURCE

\ ml v

a. HORN

60 mW 240 mW

S3 L, ill

A

STOP! exit b. SPEAKER 2.2 kHz, Intruder A

Geometry B, Position I

Instrument Shop

Figure 8.3

61
8.2.4 Detection as a Function of Geometry

The effects of microphone location upon detection at four

different carrier frequencies are shown in figure 8.4. Verylittle difference in the detection sensitivity as a function

of frequency is seen. It is apparent that a combination

of only positions one and three would produce a detection level

of greater than 10 dB for any point in the shop which the

intruder passed.

Figure 8.5 shows the results of operating the system with

the speaker at one end and using microphone position one

only (geometry C) . In this configuration detection at greater

than 10 dB occurred for the entire time the intruder was in

the shop. Positions two and three were also tested but

provided no additional detection capability.

8.2.5 Intruder Signature as a Function

of Intruder Velocity and Size

The effect of intruder velocity upon the signature was

investigated in the shop to compare with the analysis from

the auditorium (fig. 8.6). The spectral shift appears to be

virtually the same as that which occurred in the auditorium

indicating that the type of enclosure is not a factor in determining the spectrum. The only difference observed was the

loss of 10 Hz energy in the intruder signature during the

return portion of the intruder path. An examination of the

recorded data indicates a probable recorder saturation at this

point, which can destroy the higher frequencies. It appears

that the absolute magnitude of the target return is not

affected, but only the spectral distribution. This spectral

distribution can provide a powerful discrimination technique.

The results of an intruder size comparison are also

shown on figure 8.6. A reduction of signal magnitude is

shown at all three correlated frequencies. The reduction

appears to be approximately 3 dB, in general agreement with

the results in the auditorium. This coincident ally corresponds to the weight ratio of the two individuals

.

A computer spectral display is included for the first

two intrusions. The frequency responses of the signatures

appear to match very well. However, the reduction in magnitude does not show up well on the three-dimensional log scale.

62
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8.3 Con cl us ions

The tests conducted in the machine shop closely duplicate the tests performed in the auditorium. These two diverse areas produced nearly identical results with regard to

the spectral properties, velocity effects, noise analysis,

and sonification level and directivity effects.

Altering the geometry produced some excellent unexpected

results. The test conducted in geometry C (single microphone,

single speaker) produced detection throughout the entire

intruder traverse. This indicates a detection coverage

increase of an order of magnitude with respect to ultrasonic

Doppler systems.

The inability to even test 5.0 kHz, coupled with difficulties at 3.4 kHz, support the assumption that the natural

vibrations of the enclosure will limit the use of the upper

carrier frequencies to mechanically rigid confines.

No clear determination was established with regard to

target strength as a function of carrier frequency. One

test indicated lower returns from 1.0 kHz, while another

did not

.

9.0 WAREHOUSE FEASIBILITY TESTS

See figure 5.4. One set of tests was conducted in the

Cameo Building. These tests were conducted to determine if

this technique is feasible in an enclosure constructed of

very loose fitting, highly flexible material.

9 . 1 Test Descriptions

As noted in section 5.4, the Cameo Building is a warehouse for surplus equipment. The construction is not rigid,

vibrates in the wind, and has several openings which allow

birds to enter and roost. Considerable wind was blowing

during 9 of the 16 tests conducted. This provided considerable ambient noise from the rattle of the sides and the

roll-up door. Microphones 1 and 3 were attached to the wall

3 meters from the floor, while microphone 2 was attached at

4 meters up on a center support column as shown in figure 5.4.

66
The speaker was set (3 meters high) on top of the office

shown in the corner of the building, All instrumentation

was assembled in the office. The intruder path is as shown

in figure 5.4. As with all other tests, the intruder entered,

waited 5 seconds, proceeded through the traverse, and paused

for 5 seconds prior to exit.

9 . 2 Tests Results and Analysis

The tests described in this section will not be broken

down into types, because it was a feasibility study. It

should be noted that some of the tests attempted could not

be conducted because of general signal instability; however,

tests were successful at all four frequencies (1.0 , 1.7, 2.2

and 3.4 kHz). As would be expected, microphone position 3,

which is near the roll-up door on the outside wall, produced

the most problems. Position 2 was always usable, while

position 1 on the inside wall produced some problems.

The results of the microphone position test are shown

in figure 9.1. Two carrier frequencies were tested and indicated no significant differences. Position 1, in which the

microphone and speaker were separated by the maximum distance,

provided the best detection. The intruder produced signal

to noise ratios in excess of 15 dB for the entire traverse

at 1 . 7 kHz

The noise background is considerably better at 1.7 kHz,

due to the increased power levels in that test. The wind

velocity had increased considerably during the 1.7 kHz test

and increased power was needed to overcome the increased

ambient noise conditions. The number 3 position on the outside

wall indicates higher noise levels at both carrier frequencies and both correlated frequencies as a result of

the increased noise.

A complete spectral analsyis as a function of carrier

frequency was not attempted in this area; however, the 10 Hz

response for each test is shown in figure 9.1. The ratio

of the 2 to 10 Hz responses is nearly the same as that

found in the spectral comparison from prior work.

A target strength measurement shown in figure 9.2 was

conducted for three frequencies. They are of a feasibility

nature and not as rigorously controlled as those in the other

areas. The test at 1.7 kHz shows an increase in power of

67
INTRUDER SIGNATURE VS. GEOMETRY

2.2 kHz - 60 mW intruder b \ j kHz - 240 mW

20dB

Cameo Building

Figure 9.1

68
TARGET STRENGTH VS. CARRIER FREQUENCY

1.0 kHz

1.7 kHz

3.4 kHz

20 dB

START

ENTER STOP I EXIT

Power- 240 mW Position 2

2 Hz Correlation

Intruder A Cameo Building

Figure 9.2

69
about 3-4 dB over that at 1.0 kHz. The result for 3.4 kHz is

peculiar in that the peaks were considerably higher than for

1.7 kHz, but the overall average does not appear to be as great. It is possible, in this area, that the higher frequency exhibits greater shadowing from the sections created

by the shelves

.

Sonification level tests were conducted at 1.7 and

2.2 kHz after the wind had subsided. Two power levels,

60 and 240 mW, were used, and the results are shown in figure

9.3. The relative noise level decreased at the higher power

level as expected, while the intruder signature remained

approximately the same. (Some increase in the magnitude of

the indicated signature is to be expected due to the display

of signal plus noise as noted earlier.)

9 . 3 Conclusions

The results of this single feasibility test series were

exceedingly encouraging. The intruder signature appears to

be virtually the same as that of all other areas . The basic

noise assumptions were supported in this area also. An

interesting hypothesis concerning possible shadowing of

the higher carrier frequencies in large partition buildings

was suggested by the tests conducted at 3.4 kHz. This type

of area should be investigated in greater detail in followon work of inter ferometric detection techniques.

10.0 LONG TERM FALSE ALARM TESTS

During a demonstration of the work to Mr. Jack Hinde

of Sandia Laboratories, he suggested a long term (8-24 hour)

false alarm test.

The receiver for this measurement series was designed

primarily for laboratory measurements which would provide

a maximum of calibration stability, linearity, and sensitivity.

Long term tracking of the phase and magnitude of the received

signal vector was not a design requirement. The limited

phase correction rendered the units unsuitable for this type

of test. It was, nevertheless, mutually agreed to attempt

a test series. Two ten-hour tests were conducted.

70
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71
10.1 Test Descriptions

The tests were conducted at 2.2 kHz and used the 2 Hz

correlator only for a detector. No multiple correlators

or decision making logic were used to improve false alarm

rates. The 2 Hz correlator output was recorded on a strip

chart. Test 1 was conducted in room 3082 at 20 mW with the

microphone at position 2. Test 2 was conducted in the NBS

auditorium at 120 mW using geometry B and position 3

(figs . 5.1 and 5.2).

In each case, the signal to noise ratio was established

before and after the false alarm test was run. It was

greater than 20 dB in both tests. Both tests were conducted

overnight and unattended.

10.2 Test Results and Analysis

As noted, the receivers were not designed for long term

operation. As such, they provide only a finite phase correction, of ± 50°, without compromising the receiver operation.

The receivers will remain phase locked up to about ± 80°

[depending upon the particular unit), but beyond ± 50° the

receiver output contains considerable noise.

Given this operational constraint, it was feared that

phase lock would be lost completely during a period of several

hours, thus invalidating the test. Loss of phase lock did

not occur during either test.

The strip chart recording produced during these tests

is 2 meters long, making it impractical to include the recording in this report.

In both tests, the phase of the received signal apparently

approached the maximum dynamic range of the control circuit.

We assume this to be true because with the test unattended,

an absolute certainty could not be established.

In the test conducted in room 3082, the output from the

correlator showed three 6 -minute periods occurring over 30

minutes of the 4th hour of the test in which the noise level

was 10 dB. The signal-to-noise ratio still remains greater

than 10 dB. The phase servo was purposely offset to greater

than 50° to observe the result on the correlator. The correlator output was identical to that produced during the

tests

.

72
It is difficult to speculate regarding types of disturbances other than phase drift that would cause a constant

level of the type seen. Ambient noise is generally more

erratic over 6-minute time periods.

During the test in the auditorium, similar outputs from

the correlator occurred. During the first 2.5 hours of the

test, seven 5 to 7 minute periods of +10 dB noise occurred.

In addition three other outputs exceeded +16 dB during the

2.5 hours; however, none reached the level of the intruder.

The remaining 7.5 hours indicated stable noise conditions.

As noted, the tests were unattended and intrusions were

conducted both before and after to insure intruder signature

levels, without any readjustments.

10.3 Conclusions

These test results exceeded our expectations. Since

signal phase stability is considerably better than had been

anticipated, hardware design requirements can be kept simpler

than originally anticipated. Of course, the above results

will have to be positively determined during the hardware

development stage.

If further tests confirm that the perturbations experienced proved to be a phase drift rather than a false

alarm caused by the test area, it appears that the low frequency interferometr ic technique may not be subject to some

of the false alarm problems that occur in systems that are

sensitive to air currents, window shake, etc.

11.0 CONCLUSIONS

The results of the program positively confirm the

applicability of the low frequency acoustical interferometric

technique for detection of human intruders into protected

areas

11.1 Conclusion by Parameter

The particular conclusions drawn from the results of

this program are stated below.

73
11.1.1 Intruder Signature

The intruder signature frequency response is directly

proportional to the carrier frequency for a walking velocity

of 0.9 meters/sec. The response is nominally 0-15 Hz at

3.0 kHz and 0-5 Hz at 1.0 kHz. At 2.2 kHz the slope of the

response is approximately 20 dB/decade for the 1-10 Hz range.

Target strength for the intruder is -26 to -30 dB with

respect to the sound pressure level in a 1 Hz bandwidth at

all frequencies tested in the larger test areas, except for

1.0 kHz, which on some tests produced an additional -3 dB

.

Though not positively confirmed, a carrier frequency

higher than 1.0 kHz is required for consistent average target

return

.

The spectrum from the intruder signature varies with

intruder velocity. Reducing the velocity by half translates

the frequency spectrum downward.

Intruder signature is proportional to intruder size at

least within the range of test intruders used, roughly in

the ratio of their weight.

11.1.2 Background Noise

Noise is produced from two sources: ambient acoustical

energy and environmental changes which alter the multiple

transmission paths thus producing unstable signal levels.

The ultimate limiting factor for signal to noise ratio

is the environment. The effects of ambient noise can be

overcome by increasing the source level. Environmental

noise is related to carrier frequency and building structure.

Increasing carrier frequency increases environmental noise.

Tests show that the more rigid the structure, the less the

environmental noise is present. For the areas tested,

3.4 kHz appears to be the upper frequency for reliable

operational use.

Tests conducted to determine the limiting effect of

ambient noise indicate that the sonification sound pressure

level should be about 50-60 dB above the ambient noise in

a 1 Hz bandwidth. At this level the ambient noise effects

are reduced to the threshold of the environmental noise in

typical enclosures.

74
11.1.3 Geometry of the Source and Receiver

In general, the maximum perturbation from the intruder

occurred when the source and receiving microphones were

separated by the maximum dis.tance. With the exception of

the single Cameo test in which shadowing was suspected at

3.4 kHz, the results indicate that detection coverage and

magnitude for a given geometry is not a function of carrier

frequency (from 1.0-3.4 kHz). Of course, as noted in section

11.1.1, the spectral character of the signature is different

but this is independent of geometry.

11.1.4 Area of Coverage

Areas of up to 600 sq. meters (6456 sq. ft.) were

tested. Even in the largest volume, signal-to-noise ratios

of +10 dB for intruders anywhere in the area were recorded

for a single source and single receiving microphone in fixed

locations

In unobstructed rooms signal-to-noise ratios of +10 dB

detection level were produced by any of the source and receiver geometries tested. Detection signal-to-noise ratio

is generally better than +16 dB , and in a majority of the

tests was better than +20 dB . Exceptions, of course,

occurred during low sonification tests conducted to measure

ambient noise effects.

11.1.5 The Effects of Source and Microphone Characteristics

Tests were conducted using both a "high-fi" speaker

and a directional exponential horn. Except in the area

immediately in front of the source, no changes in the

intruder signal could be determined between the two.

The replacement of the microphones with a high quality

sound level meter as a receiving sensor and preamplifier indi

cated no difference in the detection characteristics. It is

probable that low cost microphones can be used for detection

hardware

75
11.1.6 General Comments

Detection signal-to-noise ratios remained high (+10 dB)

even when the test area was flooded with acoustical energy

from a radio. Tests conducted with a radio operating at

-6 dB (broadband) with respect to the source level still

provided excellent detection.

Detection at extremely low intruder velocities and

low profiles appears probable from limited exploratory tests

conducted by slow walking movement and crawling. However, a somewhat more sophisticated detector logic than that required

for normal detection would probably be needed.

11.2 Recommendations

The following recommendations are offered for future

efforts in the detection by low frequency interferometric

techniques

.

11.2.1 Minimum Velocity Effects

Additional tests should be conducted to determine the

practical lower limit for intruder velocity which still permits detection. These tests could be accomplished by modifying the present receiver and correlators to determine the

intruder spectrum below 1 Hz.

11.2.2 Additional Evaluation of Sound Pressure Fields

The sound pressure level variations for a wide variety

of test areas should be thoroughly investigated to provide

engineering specifications for hardware construction prior

to a prototype engineering model detector.

11.2.3 Design and Construction of Prototype Engineering Model

Detectors

Based upon the information in this report and that

obtained by completing the efforts described in sections

11.2.1 and 11.2.2, engineering prototype detectors should

76
be designed and constructed. These units would provide

adaptive circuitry for variable sound pressure levels,

automatic phase tracking, and would allow for the incorporation of changes as the test program suggests improvements,

such as variable logic alarm systems.

11.2.4 Detection Capability and False Alarm Tests

The engineering prototype detectors should be tested

in a large number of environments.

They should be evaluated for various intrusion techniques, including minimum velocities, minimum profiles

(crawling), sound absorbent covers, intruder size, and other

techniques suggested by security personnel

In addition, long term tests should be conducted to

determine false alarm causes and rates of occurrence.

Improvements suggested by these tests should be incorporated

into the engineering prototype models for further tests.

11.2.5 Engineering Model Detector and Test

Based upon the knowledge that has been acquired from

our testing program and the developmental improvements suggested by the prototype detectors, several engineering model

detectors should be designed and constructed. The units

would probably be of more than one type in order to meet

particular requirements. They would then be assigned to

security testing laboratories for evaluation.

ACKNOWLEDGMENTS

The author is indebted to Messrs. Robert Adair and

Douglas Kremer for their enthusiastic and dedicated efforts

which included working nights and weekends to successfully

complete this program.

REFERENCE

[1] Robert J. Ulrick, Principles of Underwater Sound for

Engineers, McGraw Hill Book Company, New York, 196 7,

Section 9.

77
APPENDIX

Certain commercial components and materials are

identified in this report in order to adequately specify the

equipment. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards

,

nor does it imply that the material or equipment identified

is necessarily the best available for the purpose.

78
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ALL AMPLIFIERS - LM30IA

F 10 1.7 2.2 3.4 50 kHz

#1 110 75 43 47 30 K •2 Selected for uniform phase shift /volt response *3 .015 OI/iF 8200 2700 1500 pF •4 6.0 3.0 I.5K 562 240 ft *5 100 43 56 82 82 K All

•6 1500 1500 1500 1000 1000 pF capacitors in uf unless noted.

Figure A.

5

83
—I' QUADRATURE HYBRID

ALL AMPLIFIERS LM30IA

• Selected for center frequency

1.0 kHz 015,/F

1 7 kHz 010

2 2 kHz .0082

3 4 kHz 3400 pF

All capacitors in yf unless noted.

8 IN PHASE

Figure A.

6

84
AMPLITUDE DETECTOR

REF INPUT

NULL SIG

9i a WV ^ 12 I 91 ^ - WV -J25 MF

X

—*\2 25^F

AMP CONTROL

7 AMP UNBALANCE METER

6 LOW PASS OUT

# 0 - 1.0,17,2 2 kHz 200K- 3.4, 5 0 kHz •200K- 10, 17, 2.2 kHz IOOK- 3.4,5.0 kHz *»*300K- 1.0, 1.7,2.2 kHz 75K- 3.4,5.0 kHz

8 HIGH PASS OUT

All capacitors In yf unless noted

Figure A. 7

85
PHASE DETECTOR

91 ft VW ^~ 12 I

25/iF

91ft — WV -J-—+ 12 £ 25/iF

100 K

REF INPUT

100 K

QUAD INPUT .05 •I00K

. I00K

i|| </W^-±-« b

0^

'I00K

i K Zero

balance

I2K

200K

* I00K vW 9 AAA-

. I00K

||—AAA-*-«

100 K

* 300K- 1.0, I 7,2.2 kHz 360K-3.4.5.0 kHz ALL AMPLIFIERS LM30IA

All capacitors in uf

unless noted.

PHASE UNBALANCE METER

PHASE CONTROL LOW PASS OUT

I 8 K vV\A-J-— 8 HIGH PASS OUT

-j- l MF

Figure A.

8

86
87

USCOMM - ERL
NBS'IMA (REV. 7-73)

U.S. DEPT. OF COMM.

BIBB Irt^DlBUI^ HAT A BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 74-364

2. Gov't Accession

INO.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

DETECTION OF HUMAN INTRUDERS BY LOW FREQUENCY SONIC

INTERFERDMETRIC TECHNIQUES

5. Publication Date Mav 1974

6. Performing Organization Code

7. AUTHOR(S)

Robert E. Stoltenberg

o. r'ertorming Urgan. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Washington, D. C. 20234

10. Project/Task/Work Unit No. 2728455

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Sandia Laboratories

U. S. Atomic Energy Commission

Alb uqtierque, New Mexico

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This report examines the theory and evaluates the results of over 200 tests

of the use of low frequency sonic interference techniques for the detection

of a human intruder in a confined area. The conclusions are that this technique

is potentially a significant, improvement over conventional methods with regard

to area coverage and minimum velocity detection.

This work examined the intruder signature and background noise with respect

to Bonification frequency, source levels, intruder size, intruder velocity, source

types, area coverage (to 692 sq. meters), and geometric position of the source

and receiver in four radically different areas.

Interference effects of the intruder signature and noise were analyzed with

respect to bandwidth, spectral content, and magnitude by both computer drawn

spectral displays, and specific frequency correlators.

17. KEY WORDS (aix to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Human detector; in terferometric technique; low frequency

acoustics.

18. AVAILABILITY fj? Unlimited It. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

1

' For Official Distribution. Do Not Release to NTIS

UNCLASSIFIED

1

1 Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD CatT No. C1J

20. SECURITY CLASS

(THIS PAGE)

22. Price

1 1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151 UNCLASSIFIED

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